You can't. Sorry. What has been stored in the index is there, and there isn't a supported way to change it.
and adding to Ayn about using the scrub command in a summary indexing search and only give access to this summary index to those not allowed to see raw data
What you COULD do is to anonymize the search results instead using the scrub
command, but as dwaddle says there's no changing the actual stored data once it's been indexed.